The present invention relates to a solid electrolytic capacitor and to a production method thereof. More specifically, the present invention relates to a solid electrolytic capacitor comprising a solid electrolyte having thereon an electrically conducting polymer having a n electron-conjugated system containing as a dopant at least one organic anion selected from (1) an alkoxy-substituted naphthalene monosulfonate anion, (2) a sulfonate anion of a heterocyclic compound, and (3) an anion of an aliphatic polycyclic compound and also relates to a production method of the capacitor. Preferably, the present invention relates to a solid electrolytic capacitor comprising a solid electrolyte additionally containing another anion having a dopant ability other than the organic anion dopant, and also relates to a production method of the capacitor.
A solid electrolytic capacitor is a device where an oxide film layer, a dielectric material, is formed on an anode substrate comprising a metal foil subjected to etching treatment, a solid semiconductor layer (hereinafter, simply referred to as a solid electrolyte) is formed as an opposing electrode outside the oxide dielectric layer and preferably an electric conductor layer such as an electrically conducting paste is further formed thereon. The device is actually used after the entire device is completely sealed by an epoxy resin or the like.
For the solid electrolyte, it has been heretofore known to use, for example, an inorganic semiconductor material such as manganese dioxide and lead dioxide, a tetracyanoquinodimethane (TCNQ) complex salt, an intrinsic electrically conducting polymer having an electric conductivity in the range of from 10xe2x88x923, to 5xc3x97103 S/cm (JP-A-1-169914 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)(U.S. Pat. No. 4,803,596)) or a xcfx80-conjugated polymer such as polyaniline (JP-A-61-239617), polypyrrole (JP-A-61-240625), polythiophene derivative (JP-A-2-15611 (U.S. Pat. No. 4,910,645)) or polyisothianaphthene (JP-A-62-118511). Many of these electrically conducting polymers, which comprise a polymer main chain having a xcfx80-conjugated repeating structural unit and a dopant contained in the polymer chain, are used as an electrically conducting polymer layer (or a polymer-type charge-transfer complex). Furthermore, in recent years, not only dopants are used singly but also they are used in combination with, for example, manganese dioxide (JP-B-6-101418 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d (U.S. Pat. No. 4,959,753)) or a filler (JP-A-9-320901).
With respect to the method for forming a solid electrolyte layer, a method of fusing and thereby forming an electrically conducting polymer layer on a dielectric layer provided on a valve-acting metal surface having a microfine void structure and a method of producing the above-mentioned electrically conducting polymer on a dielectric layer have been conventionally known. More specifically, for example, in the case of using a polymer of a 5-membered heterocyclic compound such as pyrrole or thiophene, a method of dipping an anode foil in a lower alcohol/water-based solution of a 5-membered heterocyclic compound and then dipping the anode foil in an aqueous solution having dissolved therein an oxidizing agent and an electrolyte to give rise to chemical polymerization, thereby forming an electrically conducting polymer (JP-A-5-175082), and a method of applying a 3,4-dioxyethylenethiophene monomer and an oxidizing agent each preferably in the form of a solution to an oxide coating layer of a metal foil separately differing in time or simultaneously to thereby form a solid electrolyte layer (JP-A-2-15611 (U.S. Pat. No. 4,910,645)) and JP-A-10-32145 (EP-A-820076(A2), (the term xe2x80x9cEP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published European patent applicationxe2x80x9d)) are known.
In particular, JP-A-10-32145 discloses polymers of a monomer selected from pyrrole, thiophene, furan, aniline and derivatives thereof and doped with an aromatic polysulfonic acid (e.g., naphthalene disulfonic acid) having a plurality of sulfonic acid groups in the molecular structure thereof, and also discloses a polymerization method as the production method of the polymer, where a mixed solution of the above-described polymerizable monomer and an oxidizing agent is coated and dried or an oxidizing agent is introduced and then the polymerizable monomer is introduced.
Also, JP-A-10-32145 discloses a production method using the dopant of the above-described aromatic polysulfonic acid as a constituent component of the oxidizing agent (ferric salt), stating that the solid electrolytic capacitor comprising this dopant has excellent effects on the high temperature resistance or to prevent deterioration in the static capacitance.
Furthermore, JP-B-6-82590 (U.S. Pat. No. 4,959,753) discloses a solid electrolytic capacitor containing as a dopant an alkylnaphthalene sulfonate anion substituted by one or more alkyl groups, which has excellent effects on the initial property or leakage current property.
Known examples of the oxidizing agent for use, for example, in the chemical polymerization of 5-membered heterocyclic compounds such as thiophene include iron(III) chloride, Fe(ClO4)3, organic acid iron(III) salt, inorganic acid iron(III) salt, alkyl persulfate, ammonium persulfate (hereinafter simply referred to as xe2x80x9cAPSxe2x80x9d), hydrogen peroxide, K2Cr2O7 (see, JP-A-2-15611 (U.S. Pat. No. 4,910,645)), cupric compounds and silver compounds (see, JP-A-10-32145 (EP-A-820076(A2))).
However, the capacitor comprising a solid electrolyte using the manganese dioxide is disadvantageous in that the oxide dielectric film layer is ruptured at the thermal decomposition of manganese nitrate and the impedance property is not satisfied. In the case of using lead dioxide, its effect on the environment must be taken into account. The capacitor comprising a solid electrolyte using a TCNQ complex salt has good heat fusion workability and excellent electric conductivity but the TCNQ complex salt itself has a problem in the heat resistance and accordingly, the soldering heat resistance is poorly reliable. In order to overcome these problems, an electrically conducting polymer such as polypyrrole is applied to the solid electrolyte on the surface of a dielectric by electrolytic polymerization or chemical polymerization but satisfactory results cannot be obtained with respect to the homogeneity of film, soldering heat resistance, impedance property and the like.
Demands for the production of a capacitor device having high performance are recently increasing and to cope with this tendency, further improvements are required on the material for the solid electrolyte, production method thereof, heat stability, homogeneity of the film and the like.
Under these circumstances, an object of the present invention is to provide a solid electrolytic capacitor having excellent properties satisfying the requirements with respect to the reduction in the weight and size, high capacity, high frequency property, tan xcex4, leakage current, heat resistance (reflow property), durability, etc. In particular, an object of the present invention is to provide a heat resistant solid electrolytic capacitor having excellent low impedance property and exhibiting durability in a sparking voltage test.
In order to attain the above-described objects, the present inventors have made extensive investigations on the kind, combination and content of a dopant anion in the electrically conducting polymer which works out to a solid electrolyte and as a result, they have now found that the above-described object of the present invention can be accomplished by provision of a specified organic anion in the solid electrolyte concerned in a solid electrolytic capacitor comprising opposing electrodes, one part electrode being a dielectric layer comprising a metal oxide and having a microfine structure provided on the surface of a valve-acting metal foil, and a solid electrolyte comprising an electrically conducting polymer formed on the dielectric layer.
More specifically, the present invention provides a solid electrolytic capacitor, which is compact and has high-performance, low impedance and exhibiting durability in a sparking voltage test, wherein the solid electrolyte contains as a dopant at least one organic anion selected from (1) an alkoxy-substituted naphthalene monosulfonate anion substituted by at least one linear or branched, saturated or unsaturated alkoxy group having 1 to 12 carbon atoms, (2) a sulfonate anion of a heterocyclic compound having a 5- or 6-membered heterocyclic ring (hereafter, referred to as heterocyclic sulfonate anion), and (3) an anion of an aliphatic polycyclic compound. The present invention also provides a production method of such a solid electrolytic capacitor.
The present invention provides the following:
[1] A solid electrolytic capacitor comprising an oxide dielectric film having provided thereon an electrically conducting polymer layer containing a xcfx80 electron-conjugated structure, wherein the polymer layer contains as a dopant at least one organic anion selected from (1) an alkoxy-substituted naphthalene monosulfonate anion substituted by at least one linear or branched, saturated or unsaturated alkoxy group having from 1 to 12 carbon atoms, (2) a sulfonate anion of a heterocyclic compound having a 5- or 6-membered heterocyclic ring (hereinafter referred to as a xe2x80x9cheterocyclic sulfonate anionxe2x80x9d)), and (3) an anion of an aliphatic polycyclic compound.
[2] The solid electrolytic capacitor as described in 1 above, wherein the organic anion as a dopant is an alkoxy-substituted naphthalene monosulfonate anion substituted by at least one linear or branched, saturated or unsaturated alkoxy group having from 1 to 12 carbon atoms.
[3] The solid electrolytic capacitor as described in 2 above, wherein at least one hydrogen on an aromatic ring of the alkoxy-substituted naphthalene monosulfonate is substituted by a substituent selected from a halogen atom, a nitro group, a cyano group, and a trihalomethyl group.
[4] The solid electrolytic capacitor as described in 1 above, wherein the organic anion as a dopant is an anion of heterocyclic sulfonate anion.
[5] The solid electrolytic capacitor as described in 4 above, wherein the heterocyclic sulfonate anion is an anion having heterocyclic skeleton selected from the group consisting of compounds containing a chemical structure of morpholine, piperidine, piperazine, imidazole, furan, 1,4-dioxane, benzimidazole, benzothiazolylthio, benzisoxazole, benzotriazole or benzofuran.
[6] The solid electrolytic capacitor as described in 4 above, wherein the heterocyclic sulfonate anion contains at least one alkylsulfonate substituent in the chemical structure thereof.
[7] The solid electrolytic capacitor as described in 1 above, wherein the organic anion as a dopant is an anion of an aliphatic polycyclic compound.
[8] The solid electrolytic capacitor as described in any of 1 to 7 above, wherein the organic anion is contained in an amount of from 0.1 to 50 mol % based on all the repeating structural units of the electrically conducting polymer.
[9] The solid electrolytic capacitor as described in any of 1 to 8, wherein in addition to the organic anion, a reductant anion of an oxidizing agent having a dopant ability is contained in an amount of from 0.1 to 10 mol %.
[10] The solid electrolytic capacitor as described in 9 above, wherein the reductant anion of an oxidizing agent is a sulfate ion.
[11] The solid electrolytic capacitor comprising an oxide dielectric film having provided thereon an electrically conducting polymer as described in any of 1 to 4 above, wherein the electrically conducting polymer contains a repeating structural unit represented by the following general formula I: 
(wherein the substituents R1 and R2 each independently represents any one monovalent group selected from hydrogen, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms, a linear or branched, saturated or unsaturated alkoxy group having from 1 to 6 carbon atoms, a hydroxyl group, a halogen atom, a nitro group, a cyano group, a trihalomethyl group, a phenyl group and a substituted phenyl group, R1 and R2 may be combined to each other at any position to form at least one divalent chain for forming at least one 5-, 6- or 7-membered saturated or unsaturated ring structure, X represents a hetero atom selected from S, O, Se, Te or NR3, R3 represents hydrogen, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms, a phenyl group or a linear or branched, saturated or unsaturated alkoxy group having from 1 to 6 carbon atoms, the alkyl group and the alkoxy group represented by R1, R2 or R3 may optionally contain in the chain thereof a carbonyl bond, an ether bond, an ester bond, an amide bond or an imino bond, and xcex4 is from 0 to 1).
[12] The solid electrolytic capacitor as described in 11 above, wherein the repeating structural unit represented by formula (I) is a chemical structure represented by the following general formula (II): 
(wherein the substituents R4 and R5 each independently represents hydrogen, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms or a substituent for forming at least one 5-. 6- or 7-membered heterocyclic structure containing the two oxygen elements shown in the formula by combining the linear or branched, saturated or unsaturated alkyl groups having from 1 to 6 carbon atoms to each other at any position, the ring structure formed in the scope thereof includes a chemical structure such as a substituted vinylene group and a substituted o-phenylene group, and xcex4 is from 0 to 1).
[13] A method for producing a solid electrolytic capacitor comprising an oxide dielectric film having provided thereon an electrically conducting polymer composition layer described in 1 above, the method comprising polymerizing a polymerizable monomer compound on an oxide dielectric film by an oxidizing agent, wherein the polymerizable monomer compound is a compound represented by the following general formula (III): 
(wherein R1, R2 and X are the same as defined in the general formula (I) above) and the polymerization reaction takes place in the presence of a compound capable of providing at least one organic anion selected from (1) an alkoxy-substituted naphthalene monosulfonate anion substituted by at least one linear or branched, saturated or unsaturated alkoxy group having from 1 to 12 carbon atoms, (2) a sulfonate anion of a heterocyclic compound having a 5- or 6-membered heterocyclic ring (hereinafter referred to as a xe2x80x9cheterocyclic sulfonate anionxe2x80x9d), and (3) an anion of an aliphatic polycyclic compound.
[14] The method for producing a solid electrolytic capacitor as described in 13 above, wherein the polymerizable monomer compound represented by formula (III) is a compound represented by the following general formula (IV): 
(wherein R4 and R5 are the same as defined in the general formula (II)).
[15] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and a step of dipping the metal anode foil in a solution containing an oxidizing agent and above-described organic anion.
[16] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and a step of dipping the metal anode foil in a solution containing a polymerizable monomer compound and above-described organic anion.
[17] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and then a step of dipping the metal anode foil in a solution containing a polymerizable monomer compound and above-described organic anion.
[18] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and then a step of dipping the metal anode foil in a solution containing an oxidizing agent and above-described organic anion.
[19] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and above-described organic anion and then a step of dipping the metal anode foil in a solution containing a polymerizable monomer compound.
[20] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and above-described organic anion and then a step of dipping the metal anode foil in a solution containing an oxidizing agent.
[21] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and above-described organic anion and then a process of dipping the metal anode foil in a solution containing a polymerizable monomer compound.
[22] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and above-described organic anion and then a process of dipping the metal anode foil in a solution containing an oxidizing agent.
[23] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and then a process of dipping the metal anode foil in a solution containing a polymerizable monomer compound and above-described organic anion.
[24] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and then a process of dipping the metal anode foil in a solution containing an oxidizing agent and above-described organic anion.
[25] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and above-described organic anion and then a process of dipping the metal anode foil in a solution containing a polymerizable monomer compound, followed by a step of washing and drying the metal anode foil.
[26] The method for producing a solid electrolytic capacitor as claimed in claim 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and above-described organic anion and then a process of dipping the metal anode foil in a solution containing an oxidizing agent, followed by a step of washing and drying the metal anode foil.
[27] The method for producing a solid electrolytic capacitor as claimed in claim 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing an oxidizing agent and then a process of dipping the metal anode foil in a solution containing a polymerizable monomer compound and above-described organic anion, followed by a step of washing and drying the metal anode foil.
[28] The method for producing a solid electrolytic capacitor as described in 13 or 14 above, wherein said method comprises a step of repeating multiple times a process of dipping a valve-acting metal anode foil having formed thereon an oxide dielectric film layer in a solution containing a polymerizable monomer compound and then a process of dipping the metal anode foil in a solution containing an oxidizing agent and above-described organic anion, followed by a step of washing and drying the metal anode foil.
[29] The method for producing a solid electrolytic capacitor as described in any of 13 to 28 above, wherein the organic anion is an alkoxy-substituted naphthalene monosulfonate anion substituted by at least one linear or branched, saturated or unsaturated alkoxy group having from 1 to 12 carbon atoms.
[30] The method for producing a solid electrolytic capacitor as described in 29 above, wherein at least one hydrogen on an aromatic ring of the alkoxy-substituted naphthalene monosulfonate anion is substituted by a substituent selected from a halogen atom, a nitro group, a cyano group, and a trihalomethyl group.
[31] The method for producing a solid electrolytic capacitor as described in any of 13 to 28 above, wherein the organic anion is a heterocyclic sulfonate anion.
[32] The method for producing a solid electrolytic capacitor as described in 31 above, wherein the heterocyclic sulfonate anion is an anion having at least one heterocyclic skeleton selected from the group consisting of compounds containing a chemical structure of morpholine, piperidine, piperazine, imidazole, furan, 1,4-dioxane, benzimidazole, benzothiazolylthio, benzisoxazole, benzotriazole or benzofuran.
[33] The method for producing a solid electrolytic capacitor as described in 31 above, wherein the heterocyclic sulfonate anion contains at least one alkylsulfonate substituent in the chemical structure thereof.
[34] The method for producing a solid electrolytic capacitor as described in any of 13 to 28 above, wherein the organic anion as a dopant is an anion of an aliphatic polycyclic compound.
[35] The method for producing a solid electrolytic capacitor as described in any of 13 to 34 above, wherein the oxidizing agent is a persulfate.
[36] The solid electrolytic capacitor as described in 1 above, wherein the solid electrolyte has at a portion which is of a lamellar structure.
[37] The solid electrolytic capacitor as described in 36 above, wherein the solid electrolyte having at a portion which is of a lamellar structure is formed on an outer surface of the dielectric film or on an outer surface and inside a fine pore portion thereof.
[38] The solid electrolytic capacitor as described in 36 or 37 above, wherein adjacent lamellae define an interstitial space therebetween over at least a portion of opposing surfaces thereof.
[39] The solid electrolytic capacitor as described in 36 or 37 above, wherein each lamella of the solid electrolyte constituting the lamellar structure is in the range of from 0.01 to 5 xcexcm, and the solid electrolyte layer has a total thickness in the range of from 1 to 200 xcexcm.